Surface Acoustic Wave (SAW) resonators are used in a variety of different circuitry, and are often found in mobile communication devices. Functionally, SAW resonators exploit the piezoelectric effect of a substrate to induce a mechanical strain in the device via an electrical input signal. The mechanical strain is then used to produce one or more desired electrical output signals. The piezoelectric effect is an interaction between the mechanical and electric properties of the substrate of the SAW resonator, which is usually a crystal with a high affinity for piezoelectric activity. When mechanical strain is induced in the crystal, an electric potential is produced and vice versa. Many SAW resonators use Interdigital Transducers (IDTs) to convert electrical signals into acoustic waves, and acoustic waves back into electrical signals. An input signal (e.g., a sinusuoidal input signal) provided to an IDT creates an alternating polarity between a set of interdigital electrodes, or fingers, of the IDT. Due to the piezoelectric properties of the substrate described above, the alternating polarity between the interdigital electrodes of the IDT creates a mechanical wave at the surface (i.e., a surface acoustic wave). The mechanical wave will generally propagate to another set of interdigital electrodes of the same or a different IDT, where it will cause a desired electrical signal to be produced.
FIG. 1 illustrates a conventional one-port SAW resonator 10. The SAW resonator 10 includes an IDT on the piezoelectric substrate 12. The IDT includes a first plurality of interdigital electrodes 14 connected to an input port IN and a second plurality of interdigital electrodes 16 connected to an output port OUT. In some SAW resonators, an acoustic mirror or reflector is added to prevent interference patterns or reduce insertion losses. In the SAW resonator 10 of FIG. 1, a first reflector 18 is located on the piezoelectric substrate at a first end of the IDT. Additionally, a second reflector 20 is located on the piezoelectric substrate at a second end of the IDT. The first reflector 18 and the second reflector 20 reflect the surface acoustic wave and generate a standing wave between the two reflectors.
FIG. 2 is a graph showing the admittance, conductance, and passband of the SAW resonator 10 shown in FIG. 1. The admittance and conductance values shown in the graph illustrate a resonance value of the SAW resonator 10, which is shown by the peak in the admittance and conductance.
However, the graph also shows some oscillations in the conductance of the SAW resonator 10 at frequencies lower than the resonance value. These oscillations are often referred to as rattling. This rattling may reduce the quality factor of the SAW resonator 10 as discussed below and therefore reduce the performance thereof.
FIG. 3 illustrates a Bode Q plot showing the quality factor (or Q factor) of the SAW resonator 10 shown in FIG. 1. The quality factor of a resonator is a dimensionless parameter that describes how under-damped the resonator is, as well as characterizes the bandwidth of the resonator relative to its center frequency. As shown in FIG. 3, the quality factor is fairly smooth above the resonance value of the SAW resonator 10, however, the rattling discussed above with respect to FIG. 2 is visible at frequencies lower than the resonance value. This rattling causes inefficiencies in the operation of the SAW resonator 10 and leads to losses in the bandwidth of the SAW resonator 10.
Accordingly, improved SAW resonators are needed with reduced rattling at frequencies lower than the resonance value.